The bomber, who was among a 450-strong crowd marching through the city, blew himself up as the procession reached the main square.

Chaotic scenes followed, with an angry mob starting fires and shooting into the air while others fled or lay on the ground to avoid gunfire.

Police were forced to quell unrest after the bombing, Sardar Khan, the chief of Quetta's police control room, said.

"An angry mob tried to set on fire a private building and vehicles. Some of the participants were armed and they were firing in the air. They also set on fire some bicycles and motorcycles."

Major-General Athar Abbas, a spokesman for the Pakistani military, told Al Jazeera that such attacks are taking place in urban centres because "the army has moved into large swathes of tribal areas, denying [the fighters] their freedom of movement and dislodging them from their main strongholds".

"[As a result] they have now resorted to acts of desperation, targeting our population and trying to exert pressure on the government and the military to reconsider their operations in tribal areas as they are being defeated there ... this is the phenomenon that we see."

Attack condemned

Yousuf Raza Gilani, Pakistan's prime minister, strongly condemned the bombing and called for an immediate investigation.

Washington also weighed in, condemning the targeting of civilians in Pakistan.

Attacks against religious minorities come at an already difficult time as flood aid trickles into Pakistan [AFP]

"To target innocent civilians during the Muslim holy month of Ramadan at an already difficult time as the country is working hard to recover from terrible flooding caused by monsoons makes these acts even more reprehensible," the White House said.

Friday's rally in Quetta was held to mark al-Quds day, an international event staged each year by the Shia community to oppose Israel's control of Jerusalem and show solidarity with Palestinian Muslims.

Shia Muslims are a minority in Pakistan, accounting for around a fifth of the country's 160 million population, which is dominated by Sunnis.

Fighters have launched a series of attacks as Muslims mark the final days of the holy month of Ramadan even as the country struggles to deal with massive flooding that has killed nearly 1,800 people and left millions reliant on aid handouts.

More than 3,660 people have been killed in a series of suicide attacks and bomb explosions, many of them carried out by the Taliban and other al-Qaeda-linked fighters, in Pakistan during the last three years.